In certain railway control systems, such as, in coded cab signal speed command control apparatus, it is mandatory to ensure that a component or circuit failure is incapable of endangering the safety and security of the crewmen and/or passengers and will not result in damage and/or destruction of the equipment. Thus, it is essential to exercise extreme care in designing the circuitry and in selecting the components of the electronic cab signal equipment to ensure fail-safe operation. For example, it is necessary to test and evaluate each and every foreseeable and possible malfunction that may occur in a vital system in order to guarantee reliable and safe operation. For instance, under no circumstance should the speed of a train exceed the authorized speed command received from the wayside apparatus. In practice, the car-carried receiving equipment includes a bank of frequency sensitive coding filters for determining which one of the plurality of coded speed command signals is being picked up from the wayside transmitter. The cab signal receiver also includes a speed governor which has various inputs energizable by the decoding filters and which is responsive to a frequency that is proportional to the actual speed of the vehicle. The actual speed of the vehicle is compared to the desired speed represented by coded speed command, and if the actual speed is equal to or under the desired speed, the brakes are held in their released position. It will be appreciated that each of the decoding filters is designed in such a manner that it is impossible to produce a false output even during a circuit failure. Thus, the vitality of the system is based upon the safe operation of the decoding filters. In some cab signal systems, as an added assurance that if two decoding filters are energized simultaneously, which may occur during speed command changes in some cases, then it is a customer requirement that the train not respond to either of the coded signals. In some cases, the speed governor is designed to cope with such a situation but in other systems the problem remains unresolved.